In today's radio communications networks a number of different technologies are used, such as Long Term Evolution (LTE), LTE-Advanced, 3rd Generation Partnership Project (3GPP) Wideband Code Division Multiple Access (WCDMA), Global System for Mobile communications/Enhanced Data rate for GSM Evolution (GSM/EDGE), Worldwide Interoperability for Microwave Access (WiMax), or Ultra Mobile Broadband (UMB), just to mention a few possible implementations. A radio communications network comprises Radio Base Stations (RBS) providing radio coverage over at least one respective geographical area forming a cell. The cell definition may also incorporate frequency bands used for transmissions, which means that two different cells may cover the same geographical area but using different frequency bands. User equipments (UE) are served in the cells by the respective radio base station and are communicating with respective radio base station. The user equipments transmit data over an air or radio interface to the radio base stations in uplink (UL) transmissions and the radio base stations transmit data over an air or radio interface to the user equipments in downlink (DL) transmissions.
A system separates a radio base station into main units (MU), also referred to as base band units or digital units, and remote radio units (RRUs). In this radio base station implementation a radio frequency and baseband processing equipment of the Radio base station are split in two different locations. The MU and RRU are connected via e.g. an optical network. The MUs may be centralized and located for example 10s of km from the remote radio units. The remote radio units are placed close to the radio antennas, e.g. in antenna masts. This minimizes feeder and jumper losses between antenna and remote radio units, which is often a major challenge to address in most radio communications networks, for example, to enhance the uplink capacity of mobile services. The signal processing is centralized in a MU, which offers processing resources for multiple cells, each covered by an antenna driven by a RRU. This allows a pool of processing resources to be dynamically shared among many cells, in line with the cloud computing principle, saving energy,
This system configured for remote radio units and main units is gaining significant interest and has some clear advantages, e.g. when it comes to installation of the remote radio units close to the antennas.
In some examples, the interface between the main units and remote radio units is an optical Non-Return to Zero (NRZ) signal, which is a sampled In-phase Quadrature (I/Q) air interface waveform. Sampling the air waveform makes the remote radio unit implementation relatively simple but leads to very high bitrates of the optical signal, in the order of 1.25 Gbps per antenna.
A Common Public Radio Interface (CPRI) specifies a Time Division Multiplexing (TDM) like protocol for Radio Base Station (RBS) configurations in a system configured for remote radio units and main units over a first layer. The application of CPRI between the main units and the remote radio units is static, i.e. determined as the RBS is deployed, and its configuration is only changed as part of a predetermined topology involving the main units and the remote radio units.
The CPRI defines a protocol which is used to connect a Radio Entity Control unit (REC), a Radio Entity (RE). In a typical configuration, the REC will be used in a Main Unit to control RE(s) in remote radio units. The main unit and remote radio units split in this main-remote approach may be applicable to a macro cabinet, main-remote, integrated antenna or indoor small cells.
CPRI requires accurate synchronization and latency control. Even if conventional CPRI transport is normally operated on fiber using point-to-point optical connections between MU and RRU distant less than a few hundreds of meters, there is a demand to extend its reach over geographical distances.
A typical location of remote radio units has a plurality of remote radio units and antennas. To reduce the required number of fibers connected to the location of remote radio units, the remote radio units are in some examples daisy-chained, i.e. wired together in a sequence or in a ring, to generate a multiplexed digital signal of up to 10 Gbps. 10 Gpbs may be the highest rate supported by the protocol generally used between a remote radio unit and a main unit, i.e. the CPRI. This does not provide for a desired flexibility in a connection between a remote radio unit and a main unit.
Traditional dedicated point to point links established between a limited number of MU ports and associated RRUs is inadequate to meet these new extended distance requirement. For example, the need of new installed fibers would be not sustainable as soon as the distances between RRU and associated main unit became longer than a few kilometers.